Historically, solder compounds have been tin-lead alloys. Specifically, tin-lead alloys provide a desirable combination of melting point, strength characteristics and electrical properties (including electrical conductivity), making their use a staple of the electronics industry for many decades.
However, recently concerns about the environment, human health and worker safety have begun to arise, and the industry has begun seeking replacements for conventional lead-containing solder alloys that perform the same job with comparable results. One such replacement alloy is a tin-copper alloy, which is a tin-based alloy having copper in an amount of typically about or less than 1 percent by weight. Tin-copper alloys often also include silver in an amount generally in the range of about 1 to 4 weight percent.
While these alloys have been shown to perform comparatively well or suitably (depending on the particular composition) compared to the strength and electrical properties of conventional tin-lead alloys, generally tin-copper alloys suffer from other significant disadvantages. They tend to be dull in appearance and not shiny, and the cost of silver-bearing tin-copper soldering alloys is significant.
Pure tin-lead solder alloys are generally highly reflective and shiny in appearance, and tend to appear dull, non-reflective or hazy in proportion to the concentration of impurities in the solder alloys. Therefore, historically a dull solder alloy appearance would indicate to a person of ordinary skill in the art that the alloy had been contaminated with undesirable impurities that tended to rob conventional tin-lead alloys of desirable properties. While a dull appearance in the more contemporary lead-free tin-copper alloys referred to above no longer necessarily is indicative of poor quality or undesirable impurities, nevertheless the electronics industry has expressed a strong prejudice against dull appearing solder alloys, and strongly prefers a shiny, reflective soldered surface.